Really interesting point but I know I've gotten older when he said 'my audience is middle-aged' but the way I read the chart from google is that most of his audience is between 21-34 with a tail of older than that and a steep drop off after the age of 40. That is older millennials for sure, but is that 'middle aged'?
With the average life expectancy of a male in the USA being ~75, it makes intuitive sense to call "middle age" 37 +/- some years.
The fact that middle aged is defined as 40-60 is weird in the same way that the "mid-western" United States being almost entirely east of Mississippi river is.
Middle aged traditionally is 40-60.